Digital communication receivers sample an incoming waveform and then reliably detect the sampled data. Typically, a receiver includes a Clock and Data Recovery (CDR) system to recover the clock and data from an incoming data stream. The CDR system generates a clock signal having the same frequency and phase as the incoming signal, which is then used to sample the received signal and detect the transmitted data.
The quality of the received signal is often impaired by inter-symbol interference, crosstalk, echo, and other noise. In addition, impairments in the receiver itself may further degrade the quality of the received signal. The received signal can be viewed as a well-known “data eye,” which is a superposition of a number of impaired individual signals with varying frequency components, for example, due to ISI and other noise. As the various impairments increase, the quality of an eye diagram or eye trace derived from or otherwise detected by observation of the received signal is impaired.
An eye diagram corresponds to a superposition of samples of a serial data signal over a unit interval of the data signal (i.e., the shortest time period over which the data signal can change state). An eye diagram may be generated by applying the serial data signal to the vertical input of an oscilloscope and triggering a horizontal sweep across the unit interval based on the data rate of the serial data signal. When the serial data signal corresponds to a pseudorandom data signal, the superposed samples appear on the oscilloscope display as an eye diagram with an eye opening bounded by two transition regions. Various features of the eye opening reveal information about the quality of the communications channel over which the serial data signal is transmitted. For example, a wide eye opening indicates that the serial data signal has a relatively low noise level and a relatively low bit-error rate, whereas a narrow eye opening indicates that the serial data signal has a relatively high noise level and a relatively high bit-error rate.
“Eye margining” is a technique by which the height and width, or margins, of a data eye can be measured. The eye margin of a receiver can be evaluated following the manufacturing process, or prior to deployment in a given application, to determine if the receiver satisfies one or more predefined margin criteria. If the receiver does not satisfy the one or more predefined margin criteria, the device can be rejected or one or more device parameters can be modified and the margin criteria can be reevaluated. Eye margining is often performed using a classical jitter tolerance technique and eye histogram techniques.
An eye diagram typically is evaluated based on the width of the eye opening, the height of the eye opening, and the rate of closure of the eye opening with variation of the sampling time. The width of the eye opening corresponds to the time interval over which the serial data signal can be sampled without inter-symbol interference. The height of the eye opening corresponds to a measure of the signal-to-noise ratio of the serial data signal. The rate of closure of the eye opening with variation of the sampling time indicates the sensitivity of the serial data signal to timing errors.
Various eye margining or monitoring circuits have been developed that measure one or more characteristic features of an eye diagram of a serial data signal in real-time. The measured features typically are used to correct distortions that are introduced into the serial data signal by the communication channel. For example, the frequency responses of some adaptive equalizers are optimized based on measurements of signal quality as indicated by one or more characteristic eye diagram features.
It is well known in the communications industry that electronic dispersion compensation (EDC or also known as equalization) can be used to reduce the adverse effects due to bandwidth limitations introduced in the supporting electronics or in the data carrier media. For example, the Institute of Electrical and Electronics Engineers published Standard 802.3ak on Feb. 9, 2004 (IEEE 802.3ak). The IEEE 802.3ak standard defines the physical layer and the data link layer's media access control for wired 10 Gbps Ethernet. Specifically, 10 Gbps Ethernet over a twin-axial communication media (e.g., an InfiniBand cable type) over signal transmission distances up to 15 m. The IEEE 802.3ak standard calls for both transmitter pre-emphasis and receiver equalization to support signal transmissions over copper at these distances. EDC or equalization has been implemented using feed-forward equalizers and decision-feedback equalizers.
EDC has also been applied to fiber optical based communication links. The IEEE 802.3aq standard was developed using equalization to compensate for signal dispersion introduced by the fiber media. A conventional arrangement is illustrated in FIG. 1. A receiver 10 includes a detector 20 and a controller 22 in a feedback loop. An optical signal is received along an optical path 11 at a tunable optical dispersion compensator (ODC) 12. The tunable ODC 12 operates in accordance with a control signal generated in the controller 22 and communicated on connection 23. As described in U.S. Patent Application Publication No. 2006/0067699, this first control signal adjusts the ODC 12 to control first-order dispersion in an optical signal. An adjusted version of the received optical signal is forwarded to optional optical element(s) 14 (e.g., a filter, an amplifier) for additional optical signal processing. The optical signal emerging from optical path 15 is received by the photosensitive diode 16, which communicates an electrical signal responsive to the received optical signal on connection 17 to an electronic dispersion compensator (EDC) 18. The EDC 18 operates in accordance with a second control signal generated in the controller 22 and communicated on connection 25. As described in U.S. Patent Application Publication No. 2006/0067699, this second control signal adjusts the EDC 18 to compensate for higher-order dispersion present in the converted electrical signal. The adjusted electrical signal is forwarded on connection 19 to detector 20. The detector 20 may comprise any of a number of known signal detectors or systems for detecting electrical signal characteristics including anomalies in the adjusted electrical signal. An electrical quality signal is communicated from the detector 20 to the controller on connection 21. The controller 22 utilizes one or more characteristics of the electrical quality signal to separately and independently control the ODC 12 and the EDC 18.
In a conventional feed-forward equalizer (FFE) approach, a FFE circuit consists of digital filter with M taps, where M is a positive integer. Each tap is exposed to the received signal after a certain delay and amplifies the received signal in accordance with a tap coefficient (Ci) or weight factor. The output of the FFE circuit is the sum of all tap outputs. This FFE output is forwarded to an error detector, which is applied in a feedback loop to adjust the individual tap coefficients to reduce the magnitude of the error. EDC was considered for short distance multi-mode fiber optical communication links. For example, equalization was proposed as an option in 28 Gbps fiber channel links. However, EDC, as it is defined in the industry standard, is not very effective when applied to fiber-based communication links.
As an example, in the case shown below, the 28G fiber channel standard with EDC was applied to a fiber link operating at 20.625 Gbps. A measure of the inter-symbol interference (ISI) for the channel is defined by the ratio of the eye opening (at the center) and the maximum amplitude of the eye diagram. More specifically, ISI=10*log(eye_opening_center/amplitude_maximum). As indicated in FIG. 2A, the amplitude of the original eye opening is 0.55(arbitrary unit, or a.u.), while the eye diagram maximum amplitude is 1.0(a.u.). Thus, in the illustrated example, ISI is 10*log(0.55/1.0)=2.6 dB. That is, the original input eye has an ISI or vertical closure penalty of about 2.6 dB, not including the effect of the bandwidth of individual taps. In generating the illustrated results it was assumed that the receiver has 0.75×baud rate in bandwidth (with unit of Hz).
The signal plot presented in FIG. 2B reveals an equalized or adjusted eye diagram without normalization. It was assumed that each tap in the equalizer had a much higher bandwidth than the receiver, despite the fact that it is difficult to manufacture or obtain high bandwidth components. The system applied equalization by using a three-tap FFE EDC with a 1 unit interval (UI) tap delay. The UI is the minimum time between signal level changes of a data signal. The UI is also known as the pulse time or symbol duration. The UI, which for some serial data transmission protocols (e.g., non return to zero modulation) coincides with the bit time, is the time elapsed in transmitting one bit. UI is a dimensionless measure of a definite time based on the transmitted data rate. For example, in a serial data communication with a baud rate of 2.5 Gbps, the UI is 0.4 nSec/baud.
In the proposed 32G Fiber Channel standard the FFE EDC minimizes the mean square error in the output of the equalizer at the center of the eye. This approach forces the center of the eye to have minimum spread around each of the data “1” level or the data “0” level. The following tap weights were found to be optimum: −0.15, 1, and −0.15. After equalization, the eye had a residual ISI of 0.1 dB. However, the FFE EDC introduced an additional penalty of 1.65 dB due to noise enhancement. This noise enhancement penalty included contributions from two parts: a) increased noise from other taps and b) a decrease in the amplitude of the signal. Regarding the increase in noise, assuming original white noise of a, the new noise,
                              σ          ⁢                                          ⁢          New                =                  σ          ⁢                                                                      ∑                  i                                ⁢                                                      (                                                                  C                        i                                                                    C                        n                                                              )                                    2                                                      .                                              Equation        ⁢                                  ⁢                  (          1          )                    Here Cn is the value of the reference tap weight and n is the location of the reference tap, and Ci is the tap weight of the ith tap. In this example, Cn=1, n=2, and Ci=−0.15, 1, −0.15, respectively, for i=1, 2, 3.This noise caused an additional noise penalty,
                              P          =                      10            ⁢                          log              ⁡                              (                                                      σ                    ⁢                                                                                  ⁢                    New                                    σ                                )                                                    ,                            Equation        ⁢                                  ⁢                  (          2          )                    which is 0.10 dB based on tap weights of −0.15, 1, and −0.15.
For this example, the signal amplitude was also reduced from 1 to 0.70. Consequently, the reduction in signal amplitude actually caused 10*log(0.7/1)=1.55 dB in penalty. In summary, the total center of eye sensitivity penalty in the original unequalized eye is 2.6 dB. In contrast, the total center of eye penalty after equalization using a known standard is 0.10 (residual ISI)+1.55(amplitude reduction)+0.10 (noise increase)=1.75 dB, yielding a very small 0.85 dB gain as a result of the EDC.